Pressure and force sensors are offered in a wide variety of variants. Depending upon the intended fields of deployment for the sensors the latter must be adapted to the respective conditions. Many sensors have a diaphragm that is mounted on the sensor housing and the sensor is tightly sealed by means of this connection. In deployment such diaphragms are, for example, exposed to a pressure chamber. The load generated by the pressure acts on the diaphragm, and as a result of deflection of the latter ultimately causes a signal on a measurement element, which is arranged in the housing behind the diaphragm as viewed in the direction of the pressure.
However, as soon as the diaphragm experiences a deflection that cannot be attributed to an external load acting on the latter, but is, for example, generated by thermal stress or by stresses introduced during mounting, an error signal is generated on the measurement element. In particular this can be the case for front-sealing sensors. These are located with the outer part of the front surface on the component and the seal is thereby generated. In order to exclude stresses on the front face diaphragm caused by mounting, shoulder-sealing sensors are specified. In these sensors the whole of the front face of the sensor is exposed to the load, that is to say, to the pressure chamber or the influence of a force. One example of such a sensor is specified in CH 702257. On a common surface with the housing the diaphragm is welded onto the latter.
In deployment the load in both cases acts on the central region of the diaphragm, which is deflected by the load, and also, depending upon the design, to a greater or lesser extent on the lateral region of the diaphragm, the outer edge. Behind this lateral edge region of the diaphragm, as viewed in the direction of the pressure or force, the connection between diaphragm and housing is located in such sensor designs. In order to subject the measurement element to a certain preload, this connection is often loaded in tension in the course of its manufacture. In deployment the external load applied to the diaphragm acts counter to this tension and generates, from a certain magnitude of the load, a pressure onto this connection.
In CH 394637 a sensor with a preloaded measurement element is described,
In U.S. Pat. No. 4,597,151 a pressure sensor is described whose curved diaphragm is welded onto both the central pressure pad and also a lateral supporting wall. In order to avoid asymmetric effects a second weld is applied on the inner edge of the diaphragm support, near to the start of the curvature.
When deployed certain sensors are subjected to severe short-term temperature fluctuations, for example, if their front face diaphragms are exposed to the pressure chamber in internal combustion engines. In these circumstances the diaphragm heats up quickly, the heat then flows via the connection from the diaphragm to the housing and ultimately distributes itself via further connections to other members of surrounding components until a temperature equilibrium is produced, or until a next combustion event causes the temperature of the diaphragm to rise once again.
The temperature differences between diaphragm and housing lead to errors in the measurements on the measurement element, which can only be monitored with difficulty. The heat flow from the diaphragm into the housing depends significantly on the connection between these components. The better the connection, the more heat that flows. It is problematic, however, if the thermal characteristics and/or the sensitivity of the sensors of the same build are subject to a large scatter. Then it is not possible to estimate, or thus to monitor, the behaviour of a sensor of a build series in the presence of strong temperature fluctuations.
In the prior art the connection between diaphragm and housing is achieved by means of welding, in that as a rule a laser weld seam is applied to the common outer edge between housing and diaphragm. The connection depth depends on the respective parameters of the laser equipment; however, this is subject to rapid ageing. This requires these parameters to be constantly checked and readjusted. If the connection depth is too deep and reaches the interior of the housing then the sensor is destroyed as a result of the heat penetration during the welding process. With too low a connection depth the connection is not defined, as a result of which the heat conduction from the diaphragm into the housing is insufficiently well ensured.